The invention relates to a two-piston percussion device comprising two percussion pistons alternately making a percussion motion in order to deliver an impact on a tool and a control valve for controlling the movement of the percussion pistons, where both percussion pistons are provided with a first pressure space and both percussion pistons comprise a first pressure surface facing the tool and communicating with said first pressure space, both percussion pistons comprise at the back end thereof a second pressure space and both percussion pistons comprise a second pressure surface directed backwards and with a surface area larger than that of the first pressure surface communicating with the second pressure space, whereby the pressure of the pressure fluid is constantly high in the first pressure space during the operation of the percussion device, and whereby the second pressure space of both percussion pistons is connected to the high pressure of the pressure fluid and correspondingly to the low pressure of the pressure fluid in order to alternately obtain a percussion motion and correspondingly a return motion, and control means for controlling the control valve with the percussion motion of both percussion pistons.
Percussion devices comprising several percussion pistons are known to be used in rock drilling apparatuses. Such apparatuses are known for instance from Japanese patent application No. 4-156914 and 4-186221. Furthermore, Rumanian patent 112705 discloses a percussion device including two pistons alternately delivering a percussion motion on a tool and a control valve that changes position while the pistons move at a particular stage of the movement of the pistons in such a manner that the pistons move in opposite directions in turns. In this solution, the valve is controlled so that a closed space is provided on opposite sides of the pistons in relation to one another, from where compressed air is able to flow only to one side of the valve and causing the position of the valve to change. However, such a solution is difficult to apply to hydraulic percussion devices, in which the rate of flow used and required for controlling the valve is very small and such a structure would therefore cause unnecessary efficiency losses. Removing large amounts of liquid flow also causes difficulties, as the hydraulic fluid is not compressed like air.